"Acclimatization to environmental hypoxia initiates a series of metabolic and musculocardio-respiratory adaptations that influence oxygen transport and utilization, or better still, being born and raised at altitude, is necessary to achieve optimal physical performance at altitude".

Published articles were located from searches of five major databases: Medline, Embase, Science Citation Index, Sports Discus, and Sport Scientific. Evidence to support the potentiating exercise effects of altitude acclimatization upon return to sea level is equivocal. Despite the lack of clear causality, elite athletes continue to spend considerable time and resources training at altitude, normally misled by subjective coaching opinions and the inconclusive findings of a large number of uncontrolled studies. Scientific investigation has focused on the optimization of the theoretically beneficial aspects of altitude acclimatization, which include increases in blood hemoglobin concentration, elevated buffering capacity, and improvements in the structural and biochemical properties of skeletal muscle. However, not all aspects of altitude acclimatization are beneficial; cardiac output and blood flow to skeletal muscles decrease, and preliminary evidence has shown that hypoxia in itself is responsible for a depression of immune function and increased tissue damage mediated by oxidative stress. Future research needs to focus on these less beneficial aspects of altitude training, the implications of which pose a threat to both the fitness and the health of the elite competitor.

Paul Bert was the first investigator to show that acclimatization to a chronically reduced inspiratory partial pressure of oxygen (PO2) invoked a series of central and peripheral adaptations that served to maintain adequate tissue oxygenation in healthy skeletal muscle. Those physiological adaptations have been promoted as causing the improvement in exercise performance during altitude acclimatization. However, it was not until half a century later that scientists suggested that the additive stimulus of environmental hypoxia could potentially compound the normal physiological adaptations to endurance training and accelerate performance improvements after return to sea level. That stimulated an exponential increase in scientific research, and since 1984, 22 major reviews summarized the physiological implications of altitude training for both aerobic and anaerobic performance at altitude and after return to sea level. Of those reviews, only eight specifically focused on physical performance changes after return to sea level, the most comprehensive of which was by Wolski et al. Few reviews have considered the potentially less favorable physiological responses to moderate altitude exposure, which include decreases in absolute training intensity, decreased plasma volume, depression of hemopoiesis and increased hemolysis, increases in sympathetically mediated glycogen depletion, and increased respiratory muscle work after return to sea level. In addition, there is a risk of developing more serious medical complications at altitude, which include acute mountain sickness, pulmonary edema, cardiac arrhythmias, and cerebral hypoxia. The possible implications of changes in immune function at altitude have also been largely ignored, despite accumulating evidence of hypoxia mediated immunosuppression.

Implication. In general, altitude training has been shown to improve performance at altitude, whereas no unequivocal evidence exists to support the claim that performance at sea level is improved. Theoretical advantages and disadvantages of altitude training for sea level performance have been proposed. While theory is strong and frequent, consistent evidence of a beneficial sea level effect does not exist.